There is considerable interest in exploiting natural gas (predominantly comprising methane) as a feed stock for the formation of higher hydrocarbons, not least because of finite oil reserves.
Several alternative approaches to the use of methane as a starting material have been explored. Firstly, conversion, via steam reforming, of methane to synthesis gas (CO and H2) followed by formation of methanol or hydrocarbons from catalysed reaction of synthesis gas (e.g. via the Fischer-Tropsch process). This approach may be chemically feasible but the energy requirements are considerable and commercialisation of this technology has not been widespread.
Secondly, aromatization of methane in the presence of an oxidising species has been used to form benzene and other aromatic hydrocarbons. Despite initial enthusiasm for oxidative dehydroaromatization, limitations in terms of C2 yield and selectivity have prevented commercial implementation.
Thirdly, aromatization of methane has been studied in non-oxidising conditions. Such non-oxidising conditions make the formation of benzene considerably less thermodynamically favourable than in the corresponding oxidative process. Thus, such a process is inherently difficult to perform on a scale that would be attractive for commercialisation. Molybdenum-containing aluminosilicates, particularly Mo/ZSM-5 and Mo/MCM-22, have been shown to be the most promising candidates for conversion of methane to benzene in a non-oxidative environment, but even these have significant drawbacks. In particular, reported results (L. Wang, L. Tao, M. Xie, G. Xu, J. Huang, Y. Xu, Catal. Lett. 21 (1993) 35; Y. Xu, S. Liu, L. Wang, M. Xie, X. Guo, Catal. Lett. 30 (1995) 135; F. Solymosi, A. Erdohelyi, A. Szoke, Catal. Lett. 32 (1995) 43; S. Wong, Y. Xu, W. Liu, L. Wang, X. Guo, Appl. Catal. A 136 (1996) 7; L. Chen, L. Lin, Z. Xu, T. Zhang, X. Li, Catal. Lett. 39 (1996) 169; D. Wang, J. H. Lunsford, M. P. Rosynek, Top. Catal. 3 (1996) 289; L. Wang, Y. Xu, S. Wong, W. Cui. X. Guo, Appl. Catal. A 152 (1997) 173; S. Liu, Q. Dong, R. Ohnishi, M. Ichikawa, Chem. Commun., 1997, 1445; F. Solymosi, J. Cserenyi, A. Szoke, T. Bansagi, A. Uszko, J. Catal. 165 (1997) 150; D. Wang, J. H. Lunsford, M. P. Rosynek, J. Catal. 169 (1997) 347; Y. Shu, Y. Xu, S. Wang, L. Wang, X. Guo, J. Catal. 170 (1997) 11; S. Liu, L. Wang, R. Ohnishi, m. Ichikawa, J. Catal. 181 (1999) 175 and Y. Xu, X. Bao, L. Lin, J. Catal. 216 (2003) 386) for methane conversion are low. Furthermore, poor catalyst stability, likely due at least in part to coke formation, means that commercialisation of this technology has not occurred.
Thus, at present, methane dehydroaromatization represents a potentially useful route to synthesis of higher hydrocarbons but significant problems exist with the technology such that commercialisation is not envisaged. In particular, the suppression of fast deactivation of the catalyst due to coke formation is a challenge that must be addressed before industrial application.